Fabricating strip waveguides by consolidating a rare-earth-doped layer with a laser

ABSTRACT

In order to fabricate strip monomode active optical waveguides for opticalelecommunications, a layer of vitreous soot is deposited on a substrate and is impregnated with a solution of a precursor of a rare-earth dopant, and a radiation with a wavelength comprised in an absorption band of the dopant is moved along the soot, along a trajectory corresponding to the geometrical shape desired for the guide, thereby forming a vitrified strip.

This is a divisional of application Ser. No. 07/958,687, filed on Oct.9, 1992, now U.S. Pat. No. 5,279,634.

The present invention relates to integrated optical components foroptical communications and more particularly to a method of fabricatingstrip active (i.e. capable of amplifying an optical signal) opticalwaveguides which are monomode in the wavelength range of interest foroptical communications.

BACKGROUND OF THE INVENTION

In recent years waveguides of rare-earth doped glass have proved to beparticularly attractive for optical communications, since doping givesthe guide active characteristics, permitting its use as a coherentsource, as an amplifier, etc. Not only active waveguides in fiber form,but also planar or strip waveguides, commonly referred to as "integratedoptical waveguides", have been produced. The transmission media mostwidely used in optical telecommunications systems are monomode fibersand the devices to be interfaced with such media should also bemonomode. Monomode integrated active guides are hence of specialinterest.

The use of ion exchange techniques, commonly utilized to fabricateconventional glass or silica guides, would be desirable also tomanufacture strip monomode active guides. These techniques in fact allowfabrication of low attenuation guides, with even complex geometries, ina relatively simple way, are relatively inexpensive and givereproducible results, all of which are important for industrialmanufacture. However, in the manufacture of conventional guides, ionexchange concerns monovalent ions, which have high mobility even atrelatively low temperatures, whereas manufacture of active guidesrequires substituting ions in the vitreous matrix with rare earth ions,which are trivalent. A ion exchange of this kind is very difficult toachieve. In fact at the temperatures (300°-500° C.) which are generallyused for fabricating conventional optical guides by ion exchange andwhich are not detrimental to the vitreous matrix of the substrate, rareearth ions have very low mobility, so that the concentrations necessaryfor active guide fabrication cannot be achieved in reasonable times.

For this reason, the techniques commonly used to fabricate strip activeoptical guides generally start from a vitreous substrate which isalready doped with rare earths and obtain the guiding regions on thissubstrate by exchange between alkaline ions in the glass and monovalentions intended to raise the refractive index in the region involved inthe exchange (e.g. exchange between Na+ or Li+ glass ions and K+, Ag+ions) as for a conventional guide. This technique has been described inthe paper entitled "ion-exchanged rare-earth doped waveguides" presentedby S. I. Najafi et al. at the International Congress on Glasses forOptoelectronics, Paris, 1989 and published in SPIE Proceedings, Vol.1128. pages 142 and ff. Yet this method is rather expensive, since itrequires doping the whole substrate and not only the guiding regions,and therefore it requires the use of large quantities of rare earthsalts, which are expensive per se.

A method of fabricating strip active guides allowing rare-earth ionintroduction only in the active regions is described by T. Kitagawa etal. in the article entitled "Guided-wave laser based on herbium-dopedsilica planar lightwave circuit", Electronics Letters, Vol. 27, No. 4,14 Feb. 1991. In accordance with this method, a core formed of a P₂ O₅ -SiO₂ layer into which herbium ions have been introduced is deposited ona silica substrate by flame hydrolysis deposition. The resulting core isgiven the geometric structure desired by reactive ion etching techniquesand it is covered with a silica overcladding still by a flame depositiontechnique (FHD). However said techniques produce guides with relativelyhigh attenuation.

OBJECT OF THE INVENTION

It is the object of with the present invention to provide a method ofmanufacturing strip active optical guides is provided, in which therare-earth ions are introduced only into the guiding regions andrelatively low attenuations are achieved.

SUMMARY OF THE INVENTION

The method of the invention is characterised in that according to theinventor a soot layer of the same composition as the substrate on whichthe guides must be fabricated is deposited on such a substrate, the sootis impregnated with a solution of at least a rare-earth dopantprecursor, a beam of a radiation with a wavelength which is comprised inan absorption band of the rare-earth dopant and to which the substrateis transparent is focused on the soot surface so as to vitrify the dopedsoot in the area hit by the beam, the beam having a diameter such thatthe vitrified area has a width corresponding to the width of the guidingregion of a monomode guide, and the radiation is made to scan the layerof deposited soot along a trajectory corresponding to the geometricalshape desired for the guide, whereby a vitrified strip is obtained.

Laser vitrification of a material applied to a substrate, for themanufacture of conventional strip optical guides, is described in thepaper "Planar and strip waveguides by sol-gel method and laserdensification" presented by M. Guglielmi et al. at the European Congresson Optics (ECO₄), The Hague, 12-14 Mar. 1991. According to that paper,guiding strips are obtained on a glass plate by depositing thereon aSiO₂ - TiO₂ sol-gel film, with such percentages of the two componentsthat the desired refractive index increase is obtained, and by scanningthe surface of such a film with the radiation of a CO₂ laser, operatingat a wavelength (10.6 μm) at which glass presents sufficient absorptionto cause vitrification. However, taking into account that the width ofthe vitrified strip is proportional to the wavelength of the radiationused, use of a CO₂ laser results in guiding strips 100-200 μm wide,which clearly are not monomode strips for the near infrared radiationused in optical telecommunications systems. Besides, as is known, such asource entails focusing and control difficulties.

Rare earths present on the contrary absorption bands ranging from thenear ultraviolet to the near infrared (from about 300 nm to about 1900mm according to the element) and hence the impregnated soot can bevitrified by using gas lasers (e.g. He-Cd, At, He-Ne, Kr, N₂ lasers)which have sufficient power to cause vitrification (more particularlypowers of the order of 1 watt) and emit radiation with the wavelengths,so that guiding strips of much limited width (more particularly of theorder of some micrometers), compatible with the requirement offabricating monomode guides, are easily obtained. Besides, theradiations of such lasers can be focused with the conventional opticsused with visible light. It is to be taken into account that, even ifone imagines of employing the sol-gel technique described in the paperby M. Guglielmi et al. to apply a film containing a rare-earth salt onthe substrate, which film could be vitrified by using radiations with awavelength compatible with monomode guide manufacture requirements, thesol-gel technique does not allow obtention in the guiding region ofmaterial with the desired purity, and hence the resulting guides havehigh attenuation (today about 1 dB/cm).

Of course the soot must also be doped with materials allowing therefractive index increase with respect to the substrate. A soot alreadydoped with such materials (e.g. GeO₂) can be deposited, or a solutioncontaining both rare-earth salts and refractive-index raising substances(e.g. an Al salt) can be used.

Advantageously, the soot doping solution is a solution in a non-aqueoussolvent. In this way the presence of hydroxyl groups, which causeattenuation at the wavelengths of interest for telecommunications, isreduced or even eliminated, and also refractive-index raising elements,such as aluminium, whose compounds react violently with water can beused.

The method in accordance with the invention can be implemented bydepositing and treating the soot on a substrate consisting of a plate.

Preferably, however, the substrate is a tube with polygonal, moreparticularly square, cross-section. In this case the soot is depositedon each internal face of the tube, the doped soot is vitrified byirradiating it from the outside and rotating the tube around its axis tosuccessively expose the various faces to the radiation and, afterformation of the vitrified strips on all faces and elimination of excesssoot, the different faces are separated.

By the deposition inside a tube it is then possible to fabricate aplurality of guides at the same time. Irradiation from the outside ispossible because glass is transparent to the radiations absorbed by rareearths, and hence irradiation does not damage it. Besides, depositioncan be carded out by IVPO (Internal Vapor Phase Oxidation) techniques,so that there is no danger of contamination by external agents.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the inventionwill become more readily apparent from the following description,reference being made to the accompanying highly diagrammatic drawing inwhich:

FIG. 1 is a schematic representation of an apparatus for fabricatingstrip guides inside a tube; and

FIG. 2 is a transverse sectional view of the tube.

SPECIFIC DESCRIPTION

As has been shown in the drawing, a glass tube 1 intended to form thesubstrate on which the guides are fabricated is mounted so as to rotatearound its axis on a conventional glass-making lathe of a chemical vapordeposition apparatus, the lathe being schematically indicated by chucks2. Tube 1 comprises a central element 3 with polygonal, moreparticularly square, cross-section, joined with two cylindrical elements4, 5. The central element has a length basically corresponding to thelength of the guides to be obtained, e.g. a few centimeters (5-10 cm).The two cylindrical elements are connected at their free ends toconduits (not shown) for the introduction of the reactants necessary todeposit inside the tube a layer of silica soot, possibly doped with oneor more elements intended to raise its refractive index, and for theexhaust of the volatile deposition reaction products. A heat source 6,e.g. a burner, can be moved along square-section element 3 while thetube rotates, to raise the temperature in such a zone up to a value highenough to cause deposition of the soot, but not its vitrification. Inone of the two cylindrical elements, e.g. element 4, in proximity of theconnection to the square central element, a radial conduit 7 with asuitable sealing system 8 is provided for introducing a solution ofrare-earth dopant precursors (generally a halide or a nitrate of thedopant elements) and possibly of a dopant intended to raise therefractive index (if the latter dopant is not already present in thedeposited soot) and for letting the solution flow out once the doping iscompleted. For simplicity of the drawing, the sealing systems at thetube ends have been omitted.

A laser source 9 placed in front of central tube element 3 emits aradiation at a wavelength comprised in one of the absorption bands ofthe rare earths used as dopants and sends such a radiation towards thetube, so as to vitrify the soot by irradiating it from the outside. Thesource is mounted on a support, schematically represented by block 10,allowing its displacement along the whole length of the central element,as shown by arrow F. Suitable focusing means 11, also mounted on block10, are associated with the source to form a beam of the diameterrequired to fabricate a monomode guide and to focus the beam on theinternal tube surface.

The method in accordance with the invention of fabricating activeoptical guides inside a tube 1 is carried out as follows. The case isconsidered of a silica glass tube, inside which a silica soot layer isdeposited. The reactants yielding the soot (e.g. SiCl₄ and O₂, andpossibly GeCl₄, if deposition of a soot already doped with therefractive-index raising element is desired) are introduced in aconventional manner into tube 1, whose element 3 is heated by burner 6up to a temperature of about 1500°-1600° C. A layer 12 is obtained withuniform thickness ranging from some ten to some hundred micrometers(e.g. 50-500 μm). Once the deposition is completed, a solutioncontaining one or more rare-earth salts and possibly a precursor of arefractive-index raising dopant (e.g. an aluminium salt), in case thedeposited soot is not yet doped to this aim, is slowly introduced intotube 1, sealed at its ends, and the soot is allowed to become uniformlyimpregnated. Preferably the solution is a non-aqueous solution, and sootdoping can be performed by the process described in European PatentApplication EP-A-0 372 550, published on Jun. 13, 1990 in the name ofSIP. Examples of dopant salts and non-aqueous solvents which can be usedare disclosed in that document. The method described in said applicationpermits doping to be performed without removing the tube from the lathe,thus eliminating a cause of contamination. Taking into account that suchmethod is intended for fabrication of optical fibres and not ofintegrated optical waveguides, it can be necessary to increase theconcentration of rare earth salts with respect to the values given inthe specification of that Patent Application, since in case of stripguide fabrication, the length over which the soot and the dopantsolution interact is shorter; e.g. the dopant concentration can havevalues of the order of 0.1 to 1. mol.

After impregnation, the solution is allowed to flow out, e.g. in one ofthe ways described in the filed Patent Application. This step isfollowed by dehydration with a gaseous He-Cl₂ -O₂ mixture at atemperature lower than 1000° C., the mixture being introduced andexhausted through the tube ends.

At this point the soot layer on a first face of central element 3 isirradiated with the radiation emitted by source 9, which in a practicalembodiment of the process is an Argon laser emitting at a wavelength of514 nm and having a power of about 1 watt. Such radiation is absorbed byNd and Er. Such high powers ensure vitrification of the whole thicknessof a deposited layer. Source 9 is moved along such a face according to atrajectory corresponding to the geometrical shape wanted for the guidingstrip. The tube is transparent to the wavelengths considered, and hencesoot vitrification by the laser beam does not damage the substrate. Thedisplacement speed of source 9 depends on layer thickness, on dopantconcentration and on beam power. Generally, for the concentrationsrequired by active guides, the above-mentioned thicknesses and a laserwith a power of some watts, displacement speed is higher than 1 mm/minand can be of some millimeters/minute, so that vitrification process isfast. At the end of the laser beam stroke, a vitrified strip is obtainedon the irradiated face, which strip has a refractive index higher thanthat of the substrate and a width of some ten micrometers, hence wellcompatible with guide monomodality requirements. The irradiation with alaser beam is repeated for the other three faces. Irradiation of oneface does not damage the opposite face, since the radiation is focusedexactly on the surface to be irradiated.

After obtention of the vitrified strips on the four faces, excess sootis removed by rinsing the inside of the tube e.g. with a solution of HFat 0.5%, which damages neither the strips formed nor the substrate. Thenthe central element is separated from the rest of the tube and the fourfaces are separated from each other.

As previously stated, the fabrication of the waveguides inside a tubehas the advantage that the materials are never exposed to the externalenvironment during the process, so that possible causes of contaminationare avoided. However the process conditions described apply also in caseof fabrication of guides on an isolated plate. In this case of coursemore precautions are to be taken to avoid contamination by externalelements. Besides, even though guide formation on a silica gas substratehas been described, the process can apply to substrates of differentgases, e.g. oxide gases, fluoride gases, and the like.

We claim:
 1. A method of fabricating an active strip monomode waveguideof a desired geometrical shape, comprising the steps of:(a) depositingupon a substrate for an active strip monomode waveguide, a soot layer ofsubstantially the same composition as that of said substrate; (b)impregnating said layer with a solution of at least one rare-earthdopant precursor; (c) drying said layer and transforming said precursorinto a rare-earth dopant distributed in said layer; (d) directing a beamof radiation to which said substrate is transparent and which is of awavelength in an absorption band of said rare-earth dopant, against saidlayer thereby heating and fusing said layer to form the active stripmonomode waveguide therefrom; and (e) during step (d) sweeping said beamagainst said layer with a trajectory corresponding to said desiredgeometrical shape.
 2. The method defined in claim 1 wherein saidradiation has a wavelength comprised in one of the following ranges: anear ultraviolet range, a visible light range and a near infrared range.3. The method defined in claim 2 wherein said solution also contains aprecursor of a refractive-index-raising dopant and, with drying of saidlayer, said precursor is transformed into said refractive-index-raisingdopant.
 4. The method defined in claim 2 wherein said solution is anonaqueous solution.